Hi, I know, it is hard to figure out the reason sometimes. But maybe, somebody, has a clue...

I have had two situations today, both ending with same behaviour.

First, I have set some folders to slideshow wallpapers. Then I've forgot them, so I went through some cleanup and renamed those folders set.
Usually, when an image is missing, the wallpaper slideshow is refreshed and look until a suitable file is found. But seems the changed paths make the system go crazy. Each time. There is a high (I could say to the maximum) disk activity, KDE Plasma desktop freezes, although it is possible to switch through the tasks/windows. But no desktop. And no desktop settings too, so can't clear the wallpaper paths. I've restarted, but KDE goes in the same state. I had to remove the paths manually, in the config file. Then restart. I didn't try to restart KDE manually, in the command line, although possible.
After restarting, it worked as usual.

I've checked in a Neon KDE VM with kernel 4.4 the KDE wallpapers scenario, but that behaved correctly.

Secondly, something has changed in VirtualBox regarding the CPU cores used. So, it took all real cores instead the virtual cores, so it took all the CPU. Previously, the 2 cores * 2 virtual cores each meant 4, i.e. 50% of my CPU. Now seems to need to set 2 cores out 4. That's OK. But when it took all the CPU it could, it started to behave as above, i.e. high (to maximum) disk activity. As it never ends by itself, I had to restart the PC manually, by resetting it, because it could not shutdown from the CTRL+ALT+DEL or power button. This had the effect of losing a partition of an external drive that has not even been mounted at the time and rendered all the external drives in a state that needed hard restart (power down/up) in order to become available again. I don't mean unplugging and plugging had an effect. I have recovered the partition fine with testdisk, this tool being the best rescue tool I've ever used!